In recent years PTFE has become popular as a material for seals due to its advantageous low-friction characteristics and durability. However, problems are encountered due to other physical properties of PTFE. Since PTFE is not moldable like natural or synthetic rubber, the PTFE sealing elements must be formed into the desired shape by alternate methods.
One type of PTFE seal is generally referred to as a "clamped case" seal because it includes an annular wafer of PTFE (the seal element) which is clamped between the parts of a two-part metal case. After a seal of this type is assembled, it is usually necessary to form the PTFE element into a generally frustoconical shape so that the free edge of the seal element is in a position to contact a shaft. It is also common practice to insert a sleeve member into the seal for purposes which will be described herein. This invention is directed to a device which forms the PTFE seal element into the desired shape and inserts a sleeve member into the seal.
By way of further background, the PTFE material is first formed into annular wafers that inherently are relatively rigid and have a high degree of elastic memory. In some designs a hydrodynamic surface pattern may be formed on the wafer. As will be more clearly described herein, the outer periphery of the wafer is clamped between the two parts of the metal case. The inner periphery of the wafer is free to perform the sealing function. This portion of the PTFE wafer is formed into a frustoconical shape so that the free edge of the wafer, and the hydrodynamic surface pattern, will be presented in operable orientation to the shaft. A sleeve is then inserted into the seal to maintain the frustoconical shape of the PTFE wafer until the seal is installed on a shaft. This sleeve may comprise a service sleeve which is eventually discarded or a wear sleeve which is mounted on the shaft with the seal.
More specifically, a service sleeve is a semi-rigid, cylindrical sleeve that is inserted into a PTFE seal prior to shipment to maintain the frusto-conical shape of the PTFE wafer and is used to facilitate slipping the seal over a shaft or inside a housing. The service sleeve reduces the possibility of injuring the PTFE sealing member during installation. Once the seal is in place, the service sleeve is removed, leaving the seal in position. A wear sleeve is a cylindrical metal sleeve used in repair applications and sheaths the shaft. The seal and wear sleeve are assembled to the shaft in one piece with the outer surface of the wear sleeve providing a smooth surface for engagement by the free edge, or lip portion, of the PTFE wafer.
Many seals are designed with a primary sealing lip and an auxiliary lip. The purpose of the auxiliary lip is to prevent the intrusion of foreign material between the surface of the shaft and the primary lip. Such foreign material is undesirable because it can cause undue wear of the primary sealing lip. The primary and auxiliary lips are usually oppositely directed when on the shaft with the primary lip being inwardly directed and the auxiliary lip being outwardly directed. This orientation of the primary and auxiliary lips is simple to achieve in prior art moldable materials, but is difficult to achieve with the rigid, nonmoldable PTFE seal elements.
Assembly of a PTFE seal to a wear sleeve is usually accomplished by forcing the sleeve axially through the PTFE wafer is one direction until the trailing lip snapped over the end of the sleeve. The sleeve was then moved in the opposite direction to bend the trailing lip in the opposite direction. This operation is tedious and can result in damage to the lip portions of the PTFE wafer if any burrs are present on the ends of the sleeve. Service sleeves on the other hand, are inserted into PFTE seals by folding the semi-rigid service sleeve and inserting it within the inner diameter of the PTFE sealing member. The service sleeve is then re-expanded to assume its normally cylindrical shape. This assembly technique is clumsy, imprecise and too frequently results in damage to either the service sleeve or the seal.
The present invention is directed to a tool for facilitating the forming of the PTFE element in seals and the assembly of such seals with a sleeve to overcome the problems set forth above.